1. Field of the Invention
The present invention relates to a lithographic apparatus, a method for calibrating a position of an object relative to a frame and a method of exposing a substrate using a lithographic apparatus.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In a scanning type lithographic apparatus, a mask (or patterning device) is carried by a support, also referred to as a mask table. While generating a pattern on a target portion of a substrate, the mask table performs scanning movements along a line of movement, in a single scan direction or scanning in both (i.e. opposite) directions along the line of movement. When a reversal of direction takes place, the mask table is decelerated and accelerated between the successive scanning movements. Also, the mask table is accelerated and decelerated before and after each scanning movement in a specific direction. Conventionally, the scanning movements are made with constant velocity. However, the scanning movements may also at least partly be made with varying velocity, e.g. the movements including at least part of the deceleration and/or acceleration phases.
The mask table supports or holds the mask. It holds the mask in a manner that depends on the orientation of the mask, the design of the lithographic apparatus, and other conditions, such as for example whether or not the mask is held in a vacuum environment. The mask table may include a frame or a table, for example, which may be fixed or movable as required. The mask table (and its control system) may ensure that the mask is at a desired position, for example with respect to the projection system.
The mask is coupled to the mask table through a clamp. Conventionally, the mask is coupled to the mask table through a vacuum clamp which may be implemented as one or more vacuum pads provided on the mask table, where at least a part of a circumferential area of the mask is held onto the vacuum pads. By the clamp, a normal force between adjacent surfaces of the mask and the mask table is generated, resulting in a friction between contacting surfaces of the mask and the mask table. The vacuum pads include one or more openings coupled to a gas discharge and supply system. Instead of a vacuum coupling between the mask and the mask table, other forms of couplings based on friction between the mask and the mask table are conceivable, such as electrostatic or mechanical clamping techniques to hold the mask against the mask table. During the exposure of the substrate, the mask and the substrate need to be synchronized in order to expose the pattern that is on the mask onto the correct position on the substrate. If a pattern is not projected on the appropriate position, a mismatch could occur with a further pattern that is projected onto the substrate. Such a so-called overlay-error could hinder the proper operation of the semiconductor device that is made.
In an ongoing development, increasing throughput requirements placed on lithographic apparatus lead to increasing scanning velocities. Consequently, deceleration and acceleration of the mask table increase. In the deceleration and acceleration phases, increased inertia forces act on the mask table and on the mask. These inertia forces may cause the mask or pattern device to displace relative to the mask table or support (i.e. the mask may slip relative to the mask table). In a lithographic apparatus, the synchronization between the mask and the substrate is controlled by a control unit that controls the stages to which the mask table and substrate table are mounted. This control is usually based on the position measurement of the mask table and the substrate table, not on an actual position measurement of the mask and the substrate. In such an arrangement, a displacement of the mask relative to mask table, is not detected or taken into account, resulting in the aforementioned mismatch.
When acceleration or deceleration of the mask is comparatively low, the occurring slip remains within acceptable limits and can be ignored. With increasing deceleration and acceleration levels, the slip occurring between the mask and the mask table increases and the reproducibility worsens resulting in an unacceptable mismatch between the actual position of a projected pattern and the required position.
In addition, the reticle or mask slip variation over a population of reticles or masks, which is relevant for overlay, may become considerable.
It can further be noted that the allowable mismatch between consecutive layers in a semiconductor device tends to decrease due to a decrease in the size of the pattern features.
Therefore, slip of the mask or patterning device becomes less tolerable, as such a slip may result in a position error of the pattern projected onto the substrate.